Physical vapor deposition (PVD) refers to a technology that under a vacuum condition, material source—the solid or liquid surface—is gasified into gaseous atoms or molecules, or part of them is ionized into ions, and then through the low-pressure gas (or plasma) process, a special-purpose film is deposited on the substrate surface. The physical vapor deposition method mainly includes vacuum vapor deposition, sputtering coating, arc plasma plating, ion plating, molecular beam epitaxy and the like; the method is applied not only in the deposition of metal film, alloy film, but also the deposition of compound film, ceramic film, semiconductor film, polymer film and the like; the technology process is simple, has less pollution on the environment, and saves raw materials, and produces even and dense film with strong adhesion to the substrate.
In view of the above advantages of the PVD method, and with the rapid development of the light emitting diode (LED) research, the method is widely applied in the fabrication of light emitting diodes. U.S. Patent Application Pub. No. 2013/0285065 discloses that the AlN thin film layer formed by the PVD method is flat and its roughness is less than 1 nm; that the lattice quality is good and the 002 peak width at half height is less than 200; and that on the thin film layer an n-type layer, a light emitting layer and a p-type layer and the like nitride layers are deposited by chemical vapor deposition (CVD). In the actual fabrication, the deposition of the crystal layer by the chemical vapor deposition method is greatly different from the PVD method in terms of the growth chamber environment, and the crystal layer is composed of the GaN system, and has large lattice mismatch with the AlN thin film layer, resulting in large stress between the AlN thin film layer by the PVD method and the nitride layer by the CVD method, which easily leads to poor quality of light emitting diode and low luminous efficiency.
Further, with the application of the patterned substrate technique, a pattern having a fine structure is fabricated on the planar substrate surface, and then the LED material epitaxially grows on the patterned substrate surface. The patterned interface changes the growth process of the GaN material, and restrains the defects from extending to the epitaxy surface and improves the efficiency of the quantum in the device; at the same time, the roughened GaN/substrate interface diffuses the photons emitted from the active region, so that the originally full-emitted photons have the opportunity to exit from the device so to effectively improve the light extraction efficiency. However, for epitaxially growing the LED material on the patterned substrate surface by the present conventional metal organic chemical vapor deposition (MOCVD), the depth of the pattern on the patterned substrate surface is required to be less than 2 μm, and if the depth is larger than this value, the MOCVD method cannot result in a quality epitaxial film layer; in addition, due to the characteristics of uneven surface of the patterned substrate, a thick buffer layer should be laminated in the n-type layer and the substrate in the LED structure, causing the epitaxial layer surface before the growth of the n-type layer to achieve the required smooth structure so to be conducive to the subsequent lamination of epitaxial layers; yet the thick underlayer structure produces a greater stress, resulting in a greater warping of the LED structure upon the completion of the growth, which is not conducive to the follow-up process implementation (such as splinter incurred in follow-up process) and leads to obvious difference of electric properties in different locations in a single LED structure and further the impact on the growth yield rate; further the doping concentration of the active layer is also limited due to the impact of the quality of the underlayer, therefore higher doping level cannot be obtained, thus limiting the further improvement of the voltage isoelectricity.